1. Field of the Invention
This invention concerns an image fixing method and, more particularly, it relates to an image fixing method suitable to equipment utilizing an electrophotographic process for use in copying machines, printers and facsimiles and, more in particular, color copying machines.
2. Description of the Related Art
In the electrophotographic process, fixed images are formed by way of plural steps of electrically forming latent images by various units on a light sensitive body utilizing a photoconductive material, developing the latent images by use of a toner, transferring latent toner images on the light sensitive body by way of or not by way of an intermediate transfer body to a transfer medium such as paper as toner images and then fixing the transferred images on the transfer medium. In recent years, along with development of equipment and repletion of communication networks in the information society, the electrophotographic process has been utilized not only in copying machines but also generally in office network printers, printers for personal computers, and printers for on demand printing. Then, high image quality, high operation speed, high reliability, reduction in size and weight, and energy saving performance have been demanded more and more irrespective of black and white or color printing.
Particularly, demands for coloration and energy saving performance have increased in recent years.
As a contact type fixing method used generally for the fixing method, a method of utilizing heat and pressure at fixing (hereinafter referred to as “heat pressing method”) is used generally. In the case of the heat pressing method, since the surface of a fixing member and toner images on a transfer medium are in contact with each other under pressurization, the heat efficiency is excellent and fixing can be conducted rapidly, which is particularly effective in high speed electrophotographic copying machines.
The fixing temperature performance of the toner contributes significantly to the energy saving performance, and the method of using a crystalline resin for a binder resin is excellent in the low temperature fixing property which is described, for example, in Japanese Laid Open Patent Application No. 2002-72557.
However, while the crystalline resin is melted sharply upon temperature elevation in fixing and can be fixed at a low temperature, it has a nature of less solidifying after fixing. Accordingly, it involves a problem of tending to suffer from damages by a releasing member. Further, in the both-face color printing, since molten toners in plural layers are peeled in the image peeling upon color fixing, it is more difficult than the peeling in the black single layer toner and, accordingly, it adopts a mechanism of conducting peeling preferentially to the heating side, that is, the discharging direction is nearer to the pressurizing member and, as a result, it involves a problem that images on the rear face tends to be roughened upon both-face printing.
Japanese laid Open Patent Applications Nos. H11-161079, H11-24461, H10-149044, H10-133505 and H8-314323, etc. disclose excellent inventions of heating the heating member by an external heating unit but they involve a problem that the fixing speed cannot be increased for keeping the temperature on the surface of the heating member only by external heating in a case of using existent toners of a high fixing temperature. Accordingly, a method of using the internal heating unit is predominant in the heating method for the heating member, and the external heating unit is merely an auxiliary unit that prevents lowering of temperature or an auxiliary heating unit that shortens the heating time.
However, in the method of using the internal heating unit, since a temperature gradient exists from the surface to the inside of the heating member and the internal temperature is higher relative to the surface temperature, the temperature of images upon passing the fixing nip is highest at the exit of the fixing nip. In a case of fixing the toner using a binder resin that contains a crystalline resin in this state, the toner becomes harder to solidify to possibly result in problems such as occurrence of roughing in the images, and damage liable to be caused to the images during transportation.